Defects (cracks, separations, or internal cavities) caused on the surface of a concrete structure such as tunnels and bridges are known to affect the soundness of the structure. It is thus necessary to accurately detect these defects so as to accurately judge the soundness of a structure.
Detection of defects of a structure such as cracks, separations, or internal cavities has been performed by an inspector's visual check or hammering tests, in which the inspector has to approach the structure for inspection. Therefore, increase in operation costs for preparing an environment to facilitate the aerial operation and economic opportunity loss due to traffic control required for setting the operational environment have presented a problem. In view of this, a method to enable an inspector to inspect a structure remotely is desired.
There is a method performed by image measurement has been proposed as a method to determine a structure status remotely. For example, a technique has been proposed to subject an image of a structure captured by an image-capturing device, to binarization processing with a predetermined threshold value, and detect a portion of the binarized image corresponding to a crack (PTL 1). There has been also proposed a technique to detect a cleavage caused in a structure, from the stress state of the structure (PTL 2, PTL 3). There have also been proposed a system that determines a failure of an object to be measured by automatically analyzing the captured image by using both of an infrared image-capturing device or a visible-light image-capturing device, and a laser image-capturing device (PTL 4), and a method to create a defect map from the image captured by image-capturing means that is excellent in portability (PTL 5). Furthermore, in NPL 1, a method to enhance the accuracy in detecting a crack by detecting the motion of the cracked area, in the moving image of the surface of a structure.